CSS: Introduce line-height variable

For template support.

refs #5543

public/css/icinga/base.less

@@ -41,6 +41,9 @@
 @font-size-small: 0.917em; // 11px
 @font-weight-bold: 600;
 
+// Set line-height w/o unit so that the line-height is dynamically calculated as font-size * line-height
+@line-height: 1.5;
+
 @table-column-padding: 0.333em; // 4px
 
 @vertical-padding: 0.5em; // 6px
@@ -141,8 +144,7 @@ td, th {
 
 #main > .container, #menu, #header {
   font-size: @font-size;
-  // Set line-height w/o unit so that the line-height is dynamically calculated as font-size * line-height
-  line-height: 1.5;
+  line-height: @line-height;
 }
 
 @media print {